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Yellow gunk inside motorcycle oil filter cover

I’m doing some basic restoration on a '75 Honda CB750K I picked up at a swap meet a few months ago. Took the oil filter assembly off to change filter and fluids and found the above yellow gunk inside. It’s kind of a pale yellow (puce?) paste.

I have no idea whatsoever how long it has been since any of this was touched, although the bike came with a Massachusetts plate with a May 2017 expiration date. Does anybody have any idea what this crap is and if it’s a bad sign?

It looks like there is water in the engine oil. Was this motorcycle stored outside with the air cleaner housing open where rain water may have entered the engine?

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Not that I’m aware of, although I only know the bike’s history a little ways back. I bought the bike from a dealer/mechanic shop (they were hosting the swap meet), and they stored it indoors. Since I got it home, it’s been under cover, albeit in a pole barn (in a portion of the barn enclosed on three sides with a roof over it and plastic vapor barrier underneath).

I guess some water could form in the engine from condensation alone. Then never burn off if the bike isn’t ridden much. Relatively normal to see similar snot stuff form on the bottom of the oil cap of a car engine used for really short trips in a cold climate.

Those are cool old bikes.


It has to be atmospheric water unless the motor was under water. What did the oil look like when it was drained? Does the engine turn over freely?

If that’s what you found the first time you opened the oil slinger, that’s the cleanest one I’ve ever seen. I’ve picked up old Hondas like yours and found solid layers of black tar-like stuff in there, that took scrapers and soaking in solvent to clear out.

When you rub the stuff you found between your fingers does it have a metallic aluminum color, with lots of very fine grit? That would be a big concern for bad bearings or bushings.

If it was mine I’d probably change the oil for some fresh regular oil, not synthetic, pull the spark plugs and check their color and condition, and crank it over with no compression, with a battery. I want to hear it turn to tell if it has noisy rods or mains, if it turns smoothly, etc. Don’t try to start it until you know the fuel system is clean and clear everywhere.

@jtsanders: The engine cranks with the electric starter (I tested it briefly when I got the battery in, before I started changing the oil). I haven’t tried to actually start it with gas in the tank yet; I have yet to clean out the carbs. The oil that came out of the pan looked okay; I’ve only found the gunk in the filter case so far.

@wentwest: Felt through latex gloves, the gunk has roughly the consistency of moly grease and is smooth to the touch.

Inorite? I just fell in love with it: it’s this dark turquoise color with black, white and gold racing stripe tank decals, the original 4-in-4-out pipes (a lot of the time people will have put a 4-into-1 kit on it), and somebody put hard saddlebags on it (a boon to me since I often commute on my motorcycles). 24300 miles on the odo, and I got it for $700 since it wasn’t running. And so far I haven’t found anything obviously wrong with it besides a glitchy left switch and turn signals and needing a new front brake (found a reproduction caliper to put on).

It seems like you might have hit the jackpot. As long as you know how to use your tools you can get this thing running in a few weeks.

Vehicles that are used mostly for short trips often form “mayonnaise” in the oil from emulsified water due to condensation. The engine is shut back off before it’s even warmed up. These vehicles also rust out mufflers.
I once worked for someone who drove his car to work every day even though he lived walking distance from work. It seemed he needed an annual muffler replacement.
Don’t start up the engine just to circulate the oil, if you start it, ride it, more than around the block or to the corner store and back, that’s what bicycles and shoe leather are for.

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I was just checking for water damage, like it spent some time underwater. That doesn’t seem to be the case here. I think you’ll be OK after you get though your methodical check.

Welp, aside from me not realizing the oil tank by the rider’s right hip has its own drain that had to be opened*, things are going okay. Gotta do a few more things before I’m ready to steal the plate off my usual bike register it and road test it (weather permitting).

* Lots more mayonnaise there (and thank you for that term, @B.L.E), and I wasted a liter and a half of 10W-40 Yamalube that I’d poured in. The gasket on the dipstick looked like it hadn’t been changed out since the bike was built, which was probably where the problem came from.

I doubt that’s gonna be much of a problem once you get to riding it on a frequent basis. An oil and filter change is probably all that’s required for that problem. Running the engine will generate plenty of heat to evaporate any remaining moisture in the system. I think the bike has been laid up for some time, and water vapor in the air condensing here and there has gunked up the oil system. Suggest to presume this same thing has happened to the fuel system. So don’t attempt to start or run it until you’ve drained and thoroughly cleaned the fuel tank. Otherwise the gunk will get sucked into the carb’s very small internal fuel pathways and clog them, which will just complicate the restoration process… Looks like a nice bike, and a good find, best of luck.

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Yeah, way ahead of you on the fuel system. I was working on another bike at the same time as this one and spent all of Wednesday using a POR-15 kit on that fuel tank (did you know that gasoline turns green when it’s been in a tank for four years? :sweat_smile:). Decided to skip it on the Honda because the gas that came out of the tank when I emptied it looked okay, but I washed it out thoroughly and I’ve replaced the aftermarket fuel lines and fuel filter (as well as the petcock, because it was missing the screen stem entirely), and I’m going to clean out the carbs and run Seafoam through the first couple tanks of gas.

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I’m pretty sure you will run into something else that needs attention on the way to it becoming a reliable street bike, but that’s part of the fun of bringing a old, silent bike back to life. Are the rubber parts looking OK generally? I’ve found there can be some unpleasant surprises from deteriorated bits. One bike I had just kept leaking oil out of the seals on the left side of the crankcase until I discovered the chain had broken at some time long ago and slightly bent the clutch push rod, so it wiggled in the rubber seal enough to let oil leak out. A replacement rod and everything was swell again.

These bikes got put away for some reason and after you get past all the clean up you find out what the issue was.

I’m pretty much replacing every rubber piece I touch on general principles. Among other things the fuel tank lid gasket and the oil dipstick looked like they hadn’t been changed out since the bike was built. The spark plug wires seem okay but I’m going to test them, along with the coils, with my multimeter before I put the tank back on. And I replaced the front brake hoses with a stainless steel kit from (the original rubber ones were so degraded they were pulling apart in my hand) and put a reproduction caliper and fresh pads on (original was badly rusted).

The old Honda 750’s engines were dry sump engines with the engine oil in a separate tank. There’s never a whole lot of oil inside the engine itself, a scavenge pump constantly sucks oil out of the sump and returns it back to the oil tank.